Originally Posted by
Mark Sampson
If you look at catalogs from pre-WWII, you'll see that lens makers often gave numbers to lenses in a series, as in "Dallmeyer #2" and many similar, rather than listing them by focal length, which became normal practice later.
It's probable that labeling shutters by number worked the same way, and were never meant to express compatibility between brands. For example, look at the size difference between an Ilex Acme #3 and a Copal #3. Compatibility came later, when there were few makers left.
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